They are sacs of normal spinal fluid, contained by a thin membrane called the "arachnoid" (like a spider web). These arachnoid cysts are very common, with the majority of them being found incidentally. Most of these cysts do not need any treatment whatsoever and have no effect on the normal development of the brain.

Rarely, these cysts can be large enough to put excessive pressure on the surrounding brain. If this is the case, the diagnosis can usually be made with a detailed neurological exam in the office, an imaging study to look closely at the cyst (usually an MRI and sometimes a CT scan) and occasionally an eye exam to look for evidence of pressure in the nerves.

This can be done surgically by opening holes in the "bubble" of the cyst (fenestration) and letting it drain into the normal pathways that reabsorb spinal fluid. Depending on the size and location of the cyst, different techniques may be used, including a craniotomy (removing and replacing a small piece of the skull to get to the cyst) or an endoscope (a small device that can be inserted within the cyst from a "minimally invasive" incision).

Some cysts may need to be treated with a tube to drain them into another part of the body, such as the abdomen. This kind of procedure is called a shunt.

In some cases, more than one procedure may need to be done, such as a craniotomy followed by a shunt.